Photoinduced absorption of oriented poly[1,6-di(N-carbazolyl)-2,4-hexadiyne]

We report the photoinduced absorption spectra of an oriented form of poly[1,6-di(N-carbazolyl)-2,4-hexadiyne] (polyDCHD) grown by molecular epitaxy. The polarized absorption spectra of the polyDCHD point out the high quality of the sample and the very high degree of alignment of the polydiacetylene chains. The millisecond photoinduced absorption spectrum shows:the same features already observed in polycrystalline samples assigned to triplet excitons in addition to bipolarons. The study of the dependence of the photoinduced absorptions on the polarization of the laser beam confirms this assignment. After correction for the same number of absorbed photons in both polarizations, the photoinduced signal assigned to bipolarons is stronger by pumping with the electric field of the radiation perpendicular to the chain axis direction. This fact is indicative of the interchain nature of the generation process of these photoexcited species. On the other hand, such an effect is not observed for the photoinduced triplet-triplet transition according to its intramolecular nature.